1. Field of the Invention
The present invention relates to an optical apparatus for use in the alignment of a wafer and a reticle in connection with reduction projection exposure of a circuit pattern or the like formed on the reticle to a resist coated on the wafer.
2. Description of the Prior Art
In order to improve the accuracy of reticle-wafer alignment, as disclosed in U.S. Pat. No. 4,771,180, for example, there has been proposed a method utilizing the two light flux interference technique. In this method, as shown in FIG. 17, for alignment of a wafer 110 and a reticle 120 having a circuit pattern thereon an alignment grating 121 is provided on the reticle 120. A light ray 122 illuminated from an alignment light source is directed to the alignment grating 121. The light ray 122 applied to the alignment grating 121 is diffracted by the alignment grating 121 and the diffracted light is projected onto the wafer 110 on a wafer stage 150 via an optical alignment system 130 and a reduction projection lens 140.
The optical alignment system 130, as shown in FIG. 18a, includes a pair of Fourier transform lenses 131 and 132, a spatial filter 133, and three mirrors 134, 135, and 136. This optical alignment system 130 selects .+-. primary rays only of the diffracted rays resulting from the light incident on the alignment grating 121 on the reticle 120 through the intermediary of the pair of Fourier transform lenses 131, 132 and the spatial filter 133, to cause those rays to intersect each other at post-focal points of the Fourier transform lenses 131 and 132 thereby to produce interference fringes. The rays from the optical alignment system 130 are projected onto the wafer 110 through the reduction projection lens 140.
As shown in FIG. 18b an alignment grating 111 is also provided on the wafer 110, and .+-. primary rays 123 and 124 are diffracted by the grating 111 to produce interference, resulting in interference fringes 125, which are then detected by a light sensor 160 (see FIG. 18a) via the reduction projection lens 140 and the optical alignment system 130. Any off-alignment between the grating 111 on the wafer 110 and the interference fringes 125 is detected on the basis of the interference light detected by the light sensor 160, and the wafer 110 is shifted in relation to the reticle 120 on the basis of the results of the detection in order to bring the wafer 110 and the reticle 120 into alignment. Only after such alignment, the circuit pattern on the reticle 120 is exposed for image projection onto the wafer 110.
As above mentioned, according to the method utilizing the two light flux interference technique, the grating 121 on the reticle 120 and the grating 111 on the wafer 110 are aligned together by means of two-flux interference fringes 125, and thus high precision alignment between the reticle 120 and the wafer 110 can be achieved.
However, such method requires the provision of the optical alignment system 130 between the reticle 120 and the reduction projection lens 140. The optical alignment system 130 requires a large number of optical members including a pair of Fourier transform lenses 131 and 132 and three mirrors 134-136, which makes the arrangement of the system very complicated. With such an arrangement, therefore, it is difficult to make such optical adjustments as required for chromatic aberration correction, imagery position adjustment for interference fringes 125 formed, by the grating 121 on the reticle 120, and the like. Another problem is that positions of individual optical members in the optical alignment system 130 may change with time. Moreover, since the optical alignment system 130 is fixed relative to the reticle 120, the optical alignment system 130 will limit the range of exposure for the circuit pattern 129 on the reticle 120 when the circuit pattern 129 is exposed to light for image projection onto the wafer 110.
As already mentioned, this method has an advantage that it permits high-precision alignment of the reticle 120 with the wafer 110. However, in actual semiconductor fabrication processes, circuit patterns on many different types of reticles are placed in superposed relation for exposure to light, and accordingly it is necessary to differentiate the position for the formation of the alignment grating 121 from reticle to reticle. As FIG. 19 shows, the position of the grating 121 formed on each reticle 120 is outside an exposure area 128 of the circuit pattern and is therefore limited to areas 127a, 127b to which light rays are applied for alignment, which causes a problem that circuit pattern formation on the wafer 110 is limited.
For the purpose of aligning the reticle 120 and the wafer 110 with each other, the wafer stage 150 is shifted and any change in the optical output of interference light diffracted from the grating 111 on the wafer 110 which result from the shifting is taken as a basis for effecting the required alignment. This practice poses a problem that the S/N ratio of a detection signal with respect to the interference light may drop due to some vibration caused to the wafer stage 150. Another problem with the method is low throughput.
As a technique replacing such a method, Japanese Laid-Open Patent Publication No. 64-82625 discloses a method which utilizes the optical heterodyne interference technique in aligning a mask and a wafer in a proximity exposure apparatus, such as an X-ray aligner. In this method, as shown in FIG. 20, laser beams emitted from a Zeeman laser 311 become incident on a reference grating 312. Diffracted rays from the reference grating 312 are reflected by a mirror 313 to become incident on a Fourier transform lens 314. The Fourier transform lens 314 is provided therein with a spatial filter which selects only .+-. primary light rays diffracted by the reference grating 312. The .+-. primary light rays selected by spatial filter are applied to an alignment grating 315 on a mask 316. The mask 316 is supported by a mask stage 319 above a wafer 318 mounted on a wafer stage 320 in such a manner that the mask 316 is slightly spaced apart from the wafer 318. On the wafer 318 there is provided an alignment grating in corresponding relation to the alignment grating 315 on the mask 316. The .+-. primary light rays applied to the alignment grating 315 on the mask 316 are diffracted by the alignment grating 315 to produce interference. In the same way, .+-. primary diffracted light rays applied to the alignment grating above the wafer 318 are diffracted by the alignment grating to produce interference. The respective interference light rays from the alignment gratings become incident upon photodetectors 325 and 326 via microlenses 321 and 322 and further through phase plates 323 and 324 respectively. The photodetectors 325 and 326 detect heterodyne optical beat signals of the corresponding interference light rays, the detection results being input to a phase meter 340. The phase meter 340 detects a phase difference between the respective heterodyne optical beat signals of the interference light rays and accordingly a mask stage drive circuit 341 is controlled so as to reduce the phase difference to zero, so that the mask stage 319 is shifted toward the wafer stage 320 for alignment of the mask 316 with the wafer 318.
Since this alignment method is for alignment of the mask 316 and the wafer 318 in a proximity exposure arrangement in which the mask 316 is disposed in proximity to the wafer 318, light beams for alignment purposes are subject to multiple reflection between the mask 316 and the wafer 318. When such multiple reflection phenomena occur, as shown in FIG. 21, depending upon the gap between the mask 316 and the wafer 318, the amplitude of the heterodyne beat signals applied to the photodetector for being detected thereby is considerably lowered, which may possibly result in some error in alignment between the mask 316 and the wafer 318.
According to the method, light beams are applied to the mask 316 and the wafer 318 from a particular direction so that the light beams to which the circuit pattern on the mask 316 is exposed for projection onto the wafer 318 will not be interrupted. Therefore, respective alignment gratings on the mask 316 and wafer 318 are limited in their positions, and accordingly the circuit pattern formed on the mask 316 is subject to a positional limitation.
Moreover, alignment precision with respect to the mask 316 and wafer 318 depends upon the pitch of two-beam interference fringes formed on the respective alignment gratings of the mask 316 and wafer 318. High-precision alignment of the mask 316 with the wafer 318 is not possible unless the pitch of two-beam interference fringes is 2 .mu.m or less. Therefore, a Fourier transform lens 314 having high resolving power is required, which is a disadvantageous factor from the standpoint of economy.
In contrast to such an alignment method utilizing optical heterodyne interference in the proximity exposure technique, there has been reported a method utilizing optical heterodyne interference in the reduction projection exposure technique (see Extended Abstracts of JSAP, The 36th spring meeting (1989), 1a-K-8, P.560). According to this method, as shown in FIG. 22, laser beams emitted from a He-Ne two-frequency laser 230 are switched over by a shutter 260 from beams directed toward an alignment grating 221 on a reticle 220 to beams directed toward an alignment grating 211 on a wafer 210 mounted on a wafer stage 250, and vice versa. One part of the beams to be switched over by the shutter 260 is emitted toward the alignment grating 221 on the reticle 220, while the other part of the beams is emitted toward the alignment grating 211 on the wafer 210 after passing through a slit 222 formed in a reticle 247 and further passing through a reduction projection lens 240. Then .+-. primary light beams diffracted by the respective alignment gratings 211 and 221 pass sequentially through a color correction lens 271, a lens 272, a polarization beam splitter 273, a lens 274, a polarizing plate 275, and a slit 276 until they become incident on a photoelectric conversion device 277. Alignment of the reticle 220 with the wafer 210 is carried out on the basis of the difference in phase between the respective light beams as detected by the photoelectric conversion device 277.
This method involves a problem that since beams emitted toward the alignment grating 221 on the reticle 220 and beams emitted toward the alignment grating 211 on the wafer 210 are switched over by the shutter 260 between the former and the latter, a throughput drop is likely to occur. Another problem with the method, as has been reported, is that any light having a plane of polarization unnecessary for alignment may possibly become included in the alignment light beams to cause lowered alignment precision.